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Electrochemical capacitors, also known as supercapacitors or ultracapacitors, have received much attention from research and development to industrialization, owing to their promise to deliver high levels of electrical power and offer long operating lifetimes. They are considered ideal candidates for energy storage in high-power applications. Benefiting from intensive nanoscale research in recent decades, remarkable improvements and development of supercapacitive energy storage systems have been achieved. Both the energy density and power density for supercapacitors have been substantially improved. In this review article, we endeavor to assess the profound impacts of nanoscale research on the development of supercapacitors, in terms of the substantial improvement of capacitive performance for electrode materials, and revolutionary advances in electrode and device configurations. In addition, recent progress in basic energy storage mechanisms and prototypes of supercapacitors are also reviewed, including a new kinetically-favored intercalation mechanism introduced for the first time. The review concludes with descriptions of the demonstration of already-realized practical applications of commercially-available supercapacitor devices, especially focusing on real usage in vehicles that are highly anticipated by future communities to further heighten the wide attention on clean energy storage systems.